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Amphibolic Pathway & Balance Sheet In Respiration
1.0The Respiratory Balance Sheet
Theoretical energy calculation for complete oxidation of one glucose molecule:
It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised; but in reality, this can remain only a theoretical exercise. These calculations can be made only on certain assumptions that:
(i) There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another.
(ii) The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
(iii)None of the intermediates in the pathway are utilised to synthesise any other compound.
(iv) Only glucose is being respired–no other alternative substrates are entering in the pathway at any of the intermediary stages.
Note: These kinds of assumptions are not really valid in a living system; all pathways work simultaneously and do not take place one after another; substrates enter the pathways and are withdrawn from it as and when necessary; ATP is utilised as and when needed; enzymatic rates are controlled by multiple means. Yet, it is useful to do this exercise to appreciate the beauty and efficiency of the living system in extraction and storing energy.
2.0Respiration-Amphibolic Pathway
i. Glucose is the favored substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration. Other substrates can also be respired, as has been mentioned earlier, but then they do not enter the respiratory pathway at the first step.
ii. Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to PGAL.
iii. The proteins would be degraded by proteases and the individual amino acids (after deamination) depending on their structure would enter the pathway at some stage within the Krebs’ cycle or even as pyruvate or acetyl CoA.
iv. Since respiration involves breakdown of substrates, the respiratory process has traditionally been considered a catabolic process and the respiratory pathway as a catabolic pathway. But is this understanding correct? We have discussed above, at which points in the respiratory pathway different substrates would enter if they were to be respired and used to derive energy. What is important to recognise is that it is these very compounds that would be withdrawn from the respiratory pathway for the synthesis of the said substrates. Hence, fatty acids would be broken down to acetyl CoA before entering the respiratory pathway when it is used as a substrate. But when the organism needs to synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it. Hence, the respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. Similarly, during breakdown and synthesis of protein too, respiratory intermediates form the link. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.
3.0Respiratory Quotient
The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is
called the respiratory quotient (RQ) or respiratory ratio.
The respiratory quotient depends upon the type of respiratory substrate used during respiration.
- When carbohydrates are used as substrate and are completely oxidised, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below:
- When fats are used in respiration, the RQ is less than 1. Calculations for a fatty acid, tripalmitin, if used as a substrate is shown:
- When proteins are respiratory substrates the ratio would be about 0.9.
In living organisms respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates.